Popularization of the cellular phone system has been spectacular in recent years, and improvements have been pursued in functions of the portable terminal used in the cellular phone system. As one of such improvements, a cellular phone system has been presented, which uses a dual band portable terminal to enable communications in two frequency bands at one portable terminal. Further, a cellular phone system has been presented, which uses a multifunctional portable terminal based on positioning by a global positioning system (GPS), an addition of a blue tooth (BT) etc. To configure this multifunctional cellular phone system, a function of selecting its four frequency bands becomes necessary. Besides, in order to realize such a function, a compact multiplexer is required for use in the portable terminal. Separation of the four frequency bands can be realized by cascade-connecting three diplexers. FIG. 17 shows a block diagram of such a conventional demultiplexing filter circuit element.
In the demultiplexing filter circuit element of FIG. 17, three demultiplexing filter circuits each constituted of a low-pass filter (LPF) and a high-pass filter (HPF) are connected in series as shown. Frequency pass bands of the demultiplexing filter circuits are different from one another. For example, when RF signals or frequency components (hereinafter, RF signals are carriers and signals in the frequency bands) f1 to f4 (frequency band of f1<frequency band of f2<frequency band of f3<frequency band of f4) included in the respective four frequency bands are input to the first demultiplexing filter circuit, the frequency component f1 is output from an output port of the low-pass filter side, and the remaining RF signals f2 to f4 are output from an output port of the high-pass filter side. The RF signals f2 to f4 are input to the second demultiplexing filter circuit. Here, the RF signal f2 is output from an output port of the low-pass filter side, and the remaining RF signals f3 and f4 are output from an output port of the high-pass filter side. The RF signals f3 and f4 are input to the third demultiplexing filter circuit. Here, the RF signal f3 is output from an output port of the low-pass filter side, and the RF signal f4 is output from an output port of the high-pass filter side.
However, in the aforementioned demultiplexing filter circuit where the three demultiplexing filter circuits are cascade-connected, while the four frequency bands can be separated as described above, there are some problems described below.
First, the RF signals f3 and f4 are passed through the three demultiplexing filter circuits, creating a problem of a large insertion loss in signal. Especially, when weak radio waves are dealt with as in the case of the GPS, it is necessary to reduce insertion losses as much as possible. Thus, a large insertion loss poses a serious problem. In the case of simply using three normal individual demultiplexing filter elements and connecting the elements, element dimensions are increased to interfere with miniaturization. Furthermore, if a plurality of frequency bands are close to one another, there is a problem of insufficient separation of RF signals.